Signaling



June 4, 1935. w. T. DITCHAM SIGNAIliNG Filed Feb. 5. 1952 MOW INVENTOR wmuw T. DITCHAM BY g/W ATTORNEY Patented June 4, 1935 iSIGNALIN G a William" Theodore" Dit'cham, Chelmsford, Essex,

England, 'ass'ignor to 'Radio Corporation of "America, wcorporation of Delaware Y Application February 5, 1932; Serial No. 591,089 In Great Britain February 12, 1931 V 5 Claims.

I r fllhissinvention relates toelectricali modulation sarrangementsand-more particularly tomeans for imodulating'a thermionic; valve continuous ,wave gtransmitter suitable-foruse ,radio transmission by speech, tones or the like. V

'aThere-are; many known forms of modulation systems each presenting its own particular advantagesand disadvantages. Forexample in one well known system a valve generator or-amplifier of lhighifrequency' currents is subjected to modula- :tion by varying the voltage appliedto the anode, the said anode voltage variation being commonly eaccomplishedby means of a, modulating valve connected :in parallel ,with the generating or amplifyingtvalve in conjunction with ;an iron-cored :auto-transformer or choke coil which is ;common I to the anode supplycircuit for. the two valves. Alternatively .the modulating valve may beconlnected toone windingpof an iron,cored double -windingwtransformer and the generating valve :xMraamplifying valve asthe case may be) to the :other winding of the transformer there being employed either two sources I, of anode voltage or a common source. V

o-Modulation. systems of this kind involve a ceretaimamouht of complication and loss of efficiency oif aihigh :percentage of i-modulation is. required, smote particularly if elargepowers havetobe dealt with. QMOIGOVGII' the employment. of -iron cores in tthetiaransformers or ichokes tends to produce distortion. V

These disadvantages may be avoided by ar- 'rangingsthemodulatingrvalve in series with the .generating or. amplifying .valve and varying the I: resistanceconstituted by, the, modulating ,valve by aa'pplying-ithe modulating voltages-:to the grid, of the imodulating valve; In such arrangements, awhich-may :be. termed series valve modulating arrangements, variation of the internal resistance :of themodulating valve will give rise, to a corresponding variation-in the-actual anode voltage applied to the. generating; or amplifying -valve. gSuch-series valve modulating arrangements althoughiknown and although offering theoretical advantages-as compared toqthe other systems of rmodulationarrangements hereinbefore described, thavepnot been widely utilized-owing to certain technica'l difiiculties'i. which occur. Obviously-the modulating valve may be connectedeitherson fitherhigh tension iside or on the earthed side of pthergeneratingzor,aamplifying valve. -In the 'for- Amer case the cathoderof said generating :or amplifying valve and the oscillatory circuit associated ,r -therewith can be iearthed g'but the cathode of the Emodulating :valverthe -:source 10f "negative grid.

bias voltage, and i the source; 01f audio frequency modulating voltages -must 1 be insulated for -sub'- stantially the full; potential'of the anode supply voltage, owing to the fact that; the 'modulating valveiison the high tension 'side of'thegenerating 485 'oramplifying valve. -In the-latter case, ice; when the modulating valveiseon-nected on the earth ed ,side of the generating or amplifying-*valve ethe cathode of the said modulating .valve canl be earthed and the various inputcircui ts ther eior L510 disposed in a convenientmannerbut the oscilla- =tory circuit of the, generating 'or-iampliiying valve will now be at 1 high r'potenti al and Qthis ,leads to .difficulties-due tostraycapacityqeffects. H

The principal object-of :the present invention is to provide animproved series valvemodulating V arrangement wherein these diificulties are. obvi- .ated.

According to this invention irna se'ries -.valve ,modulating :system -in which the rmodulating valve is connected inseries on the-low tension side of-a high frequencygeneratingoramplifying valve, the outputoscillatory circuitrasslooiated with the said generating or amplifyinglvalve is 1 associated therewith through condensers {01' such value that thesaidroscillatory circuit-may be connected" to earth substantially without 7 iiifecting the operating oflthe transmitter ,as a

whole. 'The coupling condensers should be'of'a "high reactance to modulating-frequencies, of low reactanceto the high frequencies 'andTtlie oscillatory circuit maythen 'beconductivelyj'con- The invention is illustrated 'in' the a company- Referring to the drawing the radio transmitter therein representedcomprises an ampn ymg "valve l whiclrreceives high frequency oscillations from a generator or other suitable-ifso1irce (not shown) said os'cillations being applied-i between grid-2 -and-cathode 3 *of 'the said amplifying valve "through-a coil 4 coupled to acoil 5 in -thesgrid -"circuit "of l. -The grid circuit of thesamplifying valve includes the usual :grid stopping. choke 5.6 35

. :and igridyleak resistance: 7 whichcare-jconnected Lin :series with :one another between; grid-2 (and .cathodei 3,'.. the choke -being adjacent-thegr-id.

I T-heigrideand cathode are coupled through-.a pair of coupling condensers 8, i 9, I one- 8, in the gr-id lead and'the other 9 in the cathodeleaxl to tbe -enids-of a tuned input circuit 5,10 whosednanode I2-of the -.amp1ifyingvalve l is "ing drawing which'jshows diagrammaticallyone F35 waybf carrying out the invention.

through a choke Hi to the source of anode potential (not shown) and is also coupled through a coupling condenser I4 to one end of a tuned out put circuit IS, IS the cathode of the amplifying valve being coupled through a similar coupling condenser l8 to a tapping point IS on the inductance IS. The end of oscillatory circuit I5, I 6 remote from the condenser I4 is coupled to the grid of the amplifying valve through a neutralizing condenser 20. The neutralizing condenser 20 and the tapping point l9 are so chosen that the effect of the internal capacity of the amplifying valve is substantially neutralized. Of course, any other known method of neutralizing from the anode circuit to thegrid or from the grid circuit to the anode may be adapted if desired.

The output circuit is coupled to a suitable utilization circuit e. g. a valve or valves of higher.

power than the valve 1 or as shown to an aerial A which is earthedat G' as also is the tapping point IS. The junction point of the grid stopping choke 6 and the grid leak 1 may, if desired, be connected through the usual by-pass condenser (not shown) to the cathode end of the grid leak which latter point is connected also'to the anode 21 of a modulating triode 22 whose cathode 23 is connected to earth and to negative terminal of the source of anode potential and across whose grid circuit are applied telephonic or other modulating potentials. Such modulating potentials are applied to the grid 21 through a suitable coupling condenser 25 the grid side of which is also connected to-the cathode of the said modulating valve through a suitable resistance 26 and bias battery 28 in series. The cathode of the amplifying valve i is heated by electrical'energy supplied through an iron *c oredtransformer 30 fed from an ordinary low frequency alternating current supply. The ends of the secondary 3| of the transformer are connected'to the ends of theoathode 3, which is bridged by a suitable resistance 32 whose center point is connected to one side of the condenser as shown so as to reduce any tone modulation due to the use of alternating cur- 'rent heating. Obviously an insulated'battery or dynamo may be employed for heating if desired,

.and'obviously also the cathode of the amplifying valve may be of the indirectly heated type.

V j Where however the cathode is heated through a transformer as illustrated, this transformer should be constructed to possess as low a capacity between primary and secondary windings and between'the secondary winding and the core as ,possible in order that high modulating frequencies shall not be by-passed to earth. 7 With this arrangement best results as regards depth of permissible modulationwill be obtained if the grid bias applied to the modulating valve -.be such that in the static or unmodulated condition the potential drop across the modulating valve is somewhat greater thanrthat across the amplifying valve. The condensers coupling the anodeand cathode of the amplifying valve to the tuned oscillatory output circuit should be of such value that their-reactance at the high frequencies is comparatively'small, while their reactance at the highest modulating frequencies employed is great. If this condition be satisfied the output oscillatory circuit may be 'earthed as described substantially without affecting the operation of the transmitter and substantially without any-appreciable distortion over the frequency spectrum normally required for high quality speech and music.

If the cathode of the amplifying valve be heated through a transformer as described and if the said transformer be of sufiicient high interwinding capacity, this said capacity may constitute the coupling condenser between the cathode and the tuned output oscillatory circuit, thus enabling a separate coupling condenser to be dispensed with. It is preferable however to employ the arrangement hereinbefore described and to make the interwinding transformer capacity as small as possible. Alternatively, of course, instead of coupling the cathode of the amplifying valve to the tuned circuit by means of a condenser 18 as illustrated, the cathode may be directly connected to the tuned circuit and a condenser inserted in the lead between the said tuned circuit and earth.

In some cases the coupling condensers 8 and 9 shown in the drawing as coupling the tuned input circuit 5 and I0 to the grid and cathode of the valve I may be dispensed with and the said tuned grid circuit directly connected to the grid and the cathode points.

Having now particularly described and ascertained the nature of my said invention andinwhat manner the same is to be performed I declare that what I claim is:--

l. Signaling means comprising a thermionic triode having an input circuit connected between the control electrode and cathode thereof, an output circuit including an inductance having one terminal connected to the'anode of said triode the other terminal connected by way of a neutralizing capacity to the control electrode, a point on said inductance being connected by way of an audible frequency blocking capacity to the cathode of said triode and means for modulating the-signals in said triode including a second triode having its anode electrode connected to the cathode of said first named tube and its cathode connected to said point on said inductance, whereby the impedance of said last named condenser is in parallel with the anode to cathode impedance of. said second named triode, and a source of modulating signals connected with the input electrodes of said second named triode.

2,.A radio or high frequency transmitter including, a high frequency oscillation relay valve having anode and cathode electrodes, and a control electrode energized by the oscillations to be relayed, means for modulating the oscillations relayed by said valve comprising, a modulating valve having anode and cathode electrodes, an alternating current output circuit tuned to the frequency of the oscillations to be relayed and connected to the anode and cathode electrodes of said relay valve through condensers, a direct current circuit connecting the anode tocathode impedances 'of said valves in series, and a circuit connecting the anode to cathode impedance of said modulating valve in parallel with one of said condensers, said one of said condensers bein of high reactance to modulating frequencies and low reactance to high frequencies, whereby said output circuit may be conductively connected to earth without affecting the operation of the transmitter.

3. A transmitter as claimed in claim 2 in which the high frequency oscillation relay valve has its interelectrode capacity neutralized to prevent any undesired feedback.

ing, a thermionic valve having its control grid and cathode electrodes energized at high frequency, an oscillation circuit comprising an inductance and a capacity in parallel, means for connecting one terminal of said oscillation circuit to the anode of said valve, a neutralizing capacity connecting the other terminal 01' said oscillation circuit to the input electrode of said valve, a capacity connecting a point on said inductance to the cathode of said valve, said capacity being of high impedance to oscillations of audible frequency, and means for modulating the oscillations relayed by said valve comprising, a thermionic modulator valve having its control grid and cathode connected to a source of modulating potentials and its anode to cathode impedance connected in parallel with said capacity, and a circuit connecting the cathode of said modulator valve to said point on said inductance and to ground.

5. A device as recited in claim 2 in which the direct current circuit connecting the anode to cathode impedances of said valves in series is adapted to be connected with a source of direct current potential, and in which said circuit is such that the potential drop across the impedance between the anode and cathode of said modulator valve is somewhat greater'than the potential drop across the impedance between the anode and cathode of said relay valve.

WILLIAM THEODORE DITCHAM. 

